A. Field of the Invention
The present invention relates to a new and improved valve for relieving pressure or vacuum within a vessel or conduit, and to a new and improved method for modifying a low pressure relief valve to relieve vacuum within a vessel within which substantial transient pressure conditions may exist.
B. Description of the Background Art
Large volume vessels typically include a pressure or vacuum relief valve to relieve pressure or vacuum that develops within the vessel thereby avoiding an unsafe condition. Typical valves employed in such vessels are disclosed in U.S. Pat. Nos. 2,971,531 and 4,091,837. The valves disclosed in these patents utilize a pallet type closure member or valve disc in low pressure relief services that subject the valve in the closed or non-vent position to maximum pressures in the order of 15 psi. U.S. Pat. No. 4,253,489, hereby incorporated by reference, also discloses a low pressure pallet type releif valve, however utilizing a magnetic latch to provide consistent pallet retention forces, and accordingly accurate relief settings.
Such low pressure relief valves have been used primarily to relieve vacuum in large pressure vessels such as petroleum storage tanks in which, due to the large volumes of the product to be vented, have rates of change of pressure which are exceedingly low. Since these relief valves were designed for applications with slow or low rates of change of pressure, the valves have not been suitable in applications experiencing rapid fluctuations from vacuum or negative relief pressure to relatively high positive operating pressure. For example, applications have arisen in large steam generating facilities requiring vacuum relief in vessels experiencing rapid changes in product pressure over the entire valve operating range. The valves of the type disclosed in the above-identified patents have been used in these rapid pressure fluctuating environments and have experienced short valve lives and are often damaged due to the rapid movement of the valve disc or pallet from full closed to full open positions. One solution has been to employ a massive pallet or valve disc to absorb the kinetic energy contained in the valve disc, but this method has proven unsuccessful in part due to the requirement for fast valve relief in most operations.